1. Technical Field
The present disclosure relates to a MEMS (Micro-Electro-Mechanical System) three-axis capacitive accelerometer.
2. Description of the Related Art
As is known, surface-micromachining techniques enable creation of MEMS structures within layers of semiconductor material, which have been deposited (for example, a layer of polycrystalline silicon) or grown (for example, a layer of epitaxial silicon) on top of sacrificial layers, which are removed via chemical etching.
MEMS sensors made with the semiconductor technology are today used in a wide range of technological sectors, thanks to the small dimensions, versatility, and contained costs. In particular, accelerometer sensors are today widely used in the automotive field, for example in airbag systems, in stability-control systems (ESP®—Electronic Stability Program) and in brake-assist systems (ABS—Antilock Brake System), or in the field of consumer electronic devices, for example in cell phones, photographic or video cameras, videogames consoles, etc.
In particular, MEMS accelerometers made with micromachining techniques comprise mobile regions (usually referred to as “rotor regions”, without this implying a movement thereof of a rotary type) suspended with respect to a substrate, and fixed regions (in general referred to as “stator regions”), fixed with respect to the same substrate and in particular to the accelerometer package. The mobile regions are connected to the substrate, directly or via interposition of appropriate coupling structures, by means of elastic biasing elements (referred to as “springs”), and are mobile as a result of the inertial effect with respect to the fixed regions along one or more axes, which constitute the axes of detection of the accelerometer, as a function of corresponding external accelerations.
Capacitive detection techniques are commonly used to determine the external acceleration acting on the sensor, as a function of the variation of capacitance defined by the capacitive coupling between the mobile regions and the fixed regions of the sensor, which form with one another capacitors with plane and parallel plates. In particular, the capacitive variation signal is processed with charge-integration techniques and amplified and filtered in a suitable manner to determine the value of the external acceleration.
In particular, three-axis accelerometer sensors have been proposed, which are able to detect components of linear acceleration acting along the three axes (x, y, and z) of a (Cartesian) system of three orthogonal axes fixed with respect to the corresponding package. These sensors comprise a micromechanical structure and moreover an appropriate electrical reading circuit, including hardware and/or software elements (for example, defined in the firmware of a processor).
Although advantageous owing to the integration in a single sensor (and in a single package) of the operations of detection along three mutually orthogonal detection axes, these sensors have in general rather large dimensions (with respect to MEMS devices, for example in the region of 1000 μm-1200 μm in the plane of horizontal extension). Even though numerous types of MEMS three-axes accelerometers, with a wide range of geometrical configurations of the corresponding micromechanical structure have in fact been proposed, none of these has proven optimized from the standpoint of their dimensions. In particular, generally these micromechanical structures require integration of an inertial mass for each detection axis (or, at most, of a first inertial mass for detecting accelerations acting in the plane of horizontal extension, and of a second inertial mass for detecting accelerations orthogonal to the same plane of horizontal extension), and moreover of the corresponding elements of elastic coupling with the substrate, which enable movement of the masses in the corresponding detection direction.